Recombination often refers to the process of mixing up parental DNA to produce offspring with genes from both parents.

I suspect that it could refer to any iterative combinatorial process.

In physics and astronomy, recombination refers to the absorption of one or more electrons by ionized atoms in a gas. In cosmology, recombination refers to an important phase of the universe's history -- the time when the early universe cooled enough to allow electromagnetic radiation to flow almost freely throughout the universe. This heralded the decoupling of matter and radiation and the point at which the cosmic microwave background as we see it today was created. Although the rough shape of the CMB spectrum was set much earlier, the faint anisotropies observed in the microwave background were generated at the time of recombination.

After atomic nuclei first formed, the cosmos was a soup of hydrogen and helium atoms, and electromagnetic radiation. The atoms and the radiation were "coupled" -- electrons could not remain attached to atoms because they'd be knocked off by the radiation, and radiation could not flow freely through the universe because it would be absorbed by atoms. This situation lasted from just a few minutes after the big bang until hundreds of thousands of years later. However, as the universe expanded it began to cool, and the average energy of a photon fell below that needed to ionize a hydrogen atom. When this happened, photons wouldn't necessarily ionize any neutral atom they encountered, and could thus begin to travel unimpeded through the universe. This is the epoch of recombination -- when electrons could finally recombine with atomic nuclei and form neutral atoms.

Recombination is considered an epoch rather than an event because it didn't occur all at once everywhere in the universe. This is because the density of matter and radiation wasn't the same throughout the universe -- it was anisotropic. These differences in density resulted in different opacities to radiation, so different parts of the universe became transparent at slightly different times.

In terms of redshift, recombination occurred on average at z = 1100 ± 100, or about 300,000 years after the big bang. (For reference, the most distant galaxies observed are at z = 6.7 or so.) This redshift is considered the "surface" which radiates the cosmic microwave background that we observe, and is known as the last scattering surface. The slight differences in the temperature of this last scattering surface represent different matter densities, and these differences say something about what the universe was like at the time of recombination. When astronomers measure the slight anisotropies in the CMB (as with the WMAP and COBE satellites) what they're really looking for are the faint ripples of matter density at the time of recombination.

I made use of the excellent while writing this.

Re*com`bi*na"tion (r?*k?m`b?*n?"sh?n), n.

Combination a second or additional time.


© Webster 1913.

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